Common-voice: Sense checks on recordings

[Omniscimus]

The app would benefit from some sense checks on recordings. If possible, both server-side (so contributors who validate recordings can focus more on non-garbage recordings) and client-side (so contributors who record can fix their recordings if they accidentally recorded something invalid). If we're not afraid of trolls, we could start with client-side and make it impossible to submit invalid recordings. Or even better, if the user insists on submitting them, pretend that they have been submitted but actually don't. (Obviously if it's client-side, a troll could disable these checks in the JavaScript.)

If we disallow contributors to submit recordings, then we should be sure that they're invalid. I think we could at least filter out the following:

• too short recordings (<1 second?)
• too long recordings (>10 seconds?)
• silent recordings (not sure how hard this is. Maybe detect volume level 0, or no change in pitch?)

Related: #301, #382.

[Omniscimus]

I'm currently working on this 👷 🚧